In the drilling of oil and gas wells, it is often necessary to place cement or some other material around the outside of casing to protect the casing and prevent movement of formation fluids behind the casing. The cement is typically mixed in a mixer at the surface and pumped down hole and around the outside of the casing. The mixing is typically done by combining the cement ingredients, typically water, cement, and other non-aqueous materials until the proper density is obtained, and then continuing to mix as much material as needed at that density while pumping down hole in a continuous process. The process has been automated by most service providers so that automatic controls maintain the proper density during mixing. Density is of importance because the resulting hydrostatic pressure must be high enough to keep pressurized formation fluids in place but not so high as to fracture a weak formation. However, density is only one of several properties important to a cement slurry. Typical slurry densities range from 14 ppg (lbs/gal.) to 20 ppg.
In recent years, more need has arisen for light-weight slurries that can be used in wells with low fracture gradients, i.e., in formations that cannot support high hydrostatic pressures. These slurries may range in weight from 11 ppg to less than the density of water, which is 8.33 ppg. One method for making light-weight slurries is to add low specific gravity non-aqueous materials such as hollow glass beads to the dry materials to decrease the density. A drawback with such slurries is that below certain densities, the ratio of non-aqueous material to water can change significantly with only minor changes in density. Changes in the non-aqueous material-to-water ratio can affect slurry viscosity, compressive strength, and other properties. In these situations, density-based control systems do not work well.
Recent developments in processes to mix these light weight slurries involve the measurement of volumes rather than density in order to ensure the proper proportion of non-aqueous materials and liquids. This is done by measuring the volume of all liquids going into and out of the mixing tub using, e.g., a volumetric flow meter and also measuring the tub level. The volume of non-aqueous materials added to the mixing tub is not measured, but rather is calculated from the liquid volume and level measurements. The amount of non-aqueous materials and liquids in the mixture can thus be determined and hence controlled. Examples of this type of system are described in U.S. patent application Ser. No. 2002/0093875 A1 and International Patent Application No. WO 02/44517 A1. A system that purports to better control the density of slurries is also described in U.S. Pat. No. 5,775,803.
While the above described volumetric mixing systems generally work well, they have the disadvantage of adding equipment and flow lines to the mixing systems. Additionally, new control algorithms are needed to monitor the measurements and control the process. In many applications, particularly offshore, space is not available for the additional equipment. These systems also become less accurate as the size of the mixing tub increases, sometimes limiting their application.